When is too young??

My son recently turned 9 and was going to his father's house for 2 weeks straight so I decided to buy him a prepaid cellphone so that we could talk whenever we wanted. Well he's back home now and I decided to go through his phone and saw that his father text him one morning bashing me (we have been separated for almost 4 years and are both in other relationships). I've been struggling if 9 is too young for a cellphone in the first place but now I'm not sure about talking to him about the text because I don't want him to feel like I betrayed his trust and start hiding things from me at such an early age and I doubt saying anything to his father would do anything except make the situation worse. Anyone else been through something like this??